This invention relates to a surface-mount capacitor for a power supply line and, in particular, to a surface-mount capacitor suitable for use in a decoupling circuit for a stabilized power supply connected to a CPU and a method of producing the same.
Development has been made of a surface-mount capacitor which is called a transmission-line element or a transmission-line noise filter and which has characteristics of both a capacitor and a filter, with a capacitance of several hundreds of microfarads, an ESR (equivalent series resistance) of 5 mΩ in a frequency band of 100 MHz, and an ESL (equivalent series inductance) of about 1 pH. The above-mentioned surface-mount capacitor comprising a unit capacitor element is particularly suitable for use in a decoupling circuit of a power supply line connected to a CPU. Such a surface-mount capacitor is disclosed, for example, in Japanese Unexamined Patent Application Publication (JP-A) No. 2004-55699.
Development is under way of a multilayer-type surface-mount capacitor formed by laminating or stacking a plurality of such unit capacitor elements and electrically connecting the unit capacitor elements in parallel so as to improve an electrostatic capacitance. The surface-mount capacitor of the type is operable as a high-performance surface-mount capacitor in a power supply line of a personal computer (PC), a server, a digital home electric appliance, telecommunication equipment, and so on which are improved towards a higher operation speed and a higher frequency. The technique of the above-mentioned surface-mount capacitor of a multilayer type is described in Japanese Patent Publication No. 2004-289142 filed by the present inventors.
Referring to FIG. 1, such a surface-mount capacitor of a multilayer type will be described. In the illustrated example, the surface-mount capacitor includes five capacitor elements. In each capacitor element, a foil-like or a sheet-like valve action metal is enlarged in surface area to produce an anode member. On the anode member, a dielectric coating film is formed. Further, a cathode portion 202 including a solid electrolyte and a conductive material is formed. Opposite ends of the anode member are separated or isolated by insulating resin layers 203 from the cathode portion 202 and serve as anode lead portions 201. To each of the anode lead portions 201, a U-shaped metal plate 204 is attached by ultrasonic welding and connected to upper and lower adjacent ones of the capacitor elements via a conductive paste 205. A lowermost one of the U-shaped metal plates 204 is connected to an anode terminal 206 via the conductive paste 205.
The cathode portion 202 of each capacitor element is connected to the cathode portions of the upper and the lower adjacent capacitor elements via the conductive paste 205. A lowermost one of the cathode portions 202 is mechanically connected to a cathode terminal 208 through a prepreg 207a provided with a hole and is electrically connected to the cathode terminal 208 through the conductive paste 205 filled in the hole.
To an uppermost one of the capacitor elements, a shielding metal plate 209 is adhered through a prepreg 207b provided with a hole. Through the conductive paste 205 filled in the hole, the cathode portion 202 of the uppermost capacitor element is electrically connected to the shielding metal plate 209.
Depending upon the intended use, further improvement in dimensional accuracy and higher reliability are required. For example, it is requested to achieve higher accuracy in external dimension, terminal dimension, and terminal-to-terminal dimension, improvement in coplanarity of terminal mount surfaces, enhancement of connection strength of terminals, reduction of warping in a reflow soldering step. Furthermore, a fillet forming portion is required in order to confirm a soldering state upon mounting. In addition, it is also required to improve the stability of an internal resistance of the cathode portion during long-term use.
In a production process of the surface-mount capacitor, there are various problems. For example, in ultrasonic welding of the anode lead portion, deformation due to thermal expansion is large so that the improvement in dimensional accuracy is not easy. Further, it is not easy to prevent protrusion or leakage of the prepreg or the conductive paste used for connection and to reduce a stress applied to the capacitor element in each step. As a whole, the production process is inevitably complicated. It is therefore difficult to reduce the number of steps and to improve the yield.